Adhesive composition for electrical PTC heating device

ABSTRACT

The invention relates to an electrically and thermally conductive adhesive composition for cementing together at least one metallized (e.g. aluminized or silvered) surface of a positive temperature coefficient (PTC) element to at least one metallic (e.g. aluminum) electrode, in the manufacture of PTC thermistor devices which are potentially capable of operating in a working temperature range not less than −55° C. to +300° C. and for at least 30,000 hours. The composition comprises, as the adhesive component, a curable silicone (e.g. silicone rubber) prepolymer, as well as finely divided silicon carbide and finely-divided silicon, and preferably also a finely-divided metallic powder.

FIELD OF THE INVENTION

The present invention relates to an electrically and thermallyconductive adhesive composition for use in the manufacture of positivetemperature coefficient (PTC) thermistor devices, to such method ofmanufacture and to the devices thereby obtained.

BACKGROUND OF THE INVENTION

Positive temperature coefficient (PTC) thermistor devices, comprisingelectrode/PTC element/electrode assemblies are well-known. Theassemblies may be held together mechanically or by adhesion. Theadhesion method has potentially the advantages of simplicity of design,ready transfer of heat at the contact areas, technologicaleffectiveness, reliability, durability and economy. Examples of therelevant prior art are as follows:

U.S. Pat. Nos. 4,177,376, 4,330,703 and 4,543,474 describe aself-regulating heating article comprising a PTC layer and at least onelayer of constant wattage output material at least partially contiguoustherewith and preferably bound thereto by adhesive, which is activatedand changes dimensions when the article is heated.

U.S. Pat. No. 4,419,564 describes use of an electrically and thermallyconductive adhesive for bonding PTC elements to metallic elements in aself-regulating electric heater for use in an early fuel evaporationsystem for automotive engines.

U.S. Pat. No. 4,689,878 describes heat-generating resistance devices,for use in a crankcase heater, such devices including a PTC heaterelement adhesively bonded between a pair of metallic electrodes by meansof an adhesive which is both thermally and electrically conductive.

In U.S. Pat. No. 4,899,032, there are described PTC ceramic resistorswhich are electrically coupled and mechanically fixed to metallic bodies(serving as current supply conduits) by an adhesive, and serve to heat aflowing medium in a heat exchanger.

U.S. Pat. No. 4,937,551 describes a PTC device wherein the PTC materialis protected from degradation by mechanical (e.g. shear) forces by oneor more adhesive masses in contact with both terminals of the device.

In U.S. Pat. No. 4,977,309, there is described an organic PTC devicewhich includes an organic PTC thermistor sheet, on one main surface ofwhich a pair of electrodes is formed and conductive adhesives are usedto adhere a conductive sheet at each electrode, the assembly beingcovered by insulating film.

U.S. Pat. No. 5,239,163 describes an automobile air heater utilizing PTCtablets adhesively fixed to tubular heat sinks by means of a flexibleelectrically conductive adhesive.

U.S. Pat. No. 5,354,969 describes a PTC thermistor heater in which anelectrode paste material containing conductive particles is printed andthen baked on the surface of a PTC element to form a first electrodehaving a rough surface, which is brought into contact with a secondelectrode of heat radiating means by the conductive particles.

In U.S. Pat. No. 5,358,793, there is described a device having a PTCmaterial bound on opposed sides thereof to conductive foils by aconductive adhesive having a thermal coefficient of expansionintermediate between that of the foil and that of the PTC material.

U.S. Pat. No. 5,499,087 describes a heat fixing device (forelectrophotography) including a heating head formed by attaching aradiant plate to the radiant surface of a PTC element by use of athermally conductive adhesive.

The entire contents of all of the above-stated U.S. Patents are deemedincorporated by reference herein.

In order to achieve the potential advantages mentioned above, however,it is desirable for the PTC thermistor devices to meet the followingrequirements:

1. Working temperature range is not less than about −55° C. to +300° C.

2. High thermal conductivity.

3. High thermal conductivity at the electrode/PTC element contact areas.

4. High dielectric strength and volume resistivity beyond the contactareas.

5. High resistance to repeated mechanical and thermal impacts in therange −55° C. to +300° C.

6. After hardening, adhesive resilience is maintained at the contactareas in order to compensate for mechanical tension at hightemperatures.

7. Lack of corrosive attack on the cemented PTC thermistor devices.

8. The above-mentioned qualities 1-7 should be maintained for no lessthan 30,000 hours.

Known adhesives exhibit some but not all of these desiredcharacteristics. For example, the high-temperature adhesive Ceramobond™5526, AREMCO PRODUCTS INC. Catalog M12 5/95, Bulletin No, M2, complieswith the requirements as to working temperature range, dielectricproperties and resistance to mechanical/thermal stresses (1, 4, 5), butdoes not comply with the other parameters (2, 3, 6, 8). Other adhesives,for example, AREMKO-BOND™ EPOXY 805. AREMCO PRODUCTS INC. Catalog V125/95, Bulletin No, M6, or AREMCO-SHlELD™ 615, AREMCO PRODUCTS INC.Catalog M12 5,95, Bulletin No. M17, comply with the above requirementsas to working temperature range, heat conductivity and electricalconductivity/dielectric properties (1, 2, 4, 7), but not with the otherparameters (3, 5, 6, 8). Some silicone adhesives, for example, DOWCORNING™ 3145 Silicone Adhesive-Gray, DOW CORNING CORPORATION, complywith the requirements as to working temperature range, dielectricproperties, and resilience after hardening (1, 4, 5, 6, 7) but not withthe other parameters (2, 3, 8). It is clear that none of the knownadhesives combine all of the above-stated requirements, and inparticular that none of them combine properties 3 and 8 with the otherparameters, i.e. especially they do not provide high efficiency,reliability and durability in full measure.

A principal object of the present invention is to make possible theprovision of an electrically and thermally conductive adhesivecomposition for cementing together PTC elements having metallizedsurfaces to metallic electrode elements, which combines all of thedesired characteristics.

Another object of the invention is make possible the provision of suchan adhesive which decreases resistance to thermal transfer and therebyincreases the heat output of the resulting PTC thermistor devices.

Other objects of the invention will appear from the ensuing description.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides, in an electrically andthermally conductive adhesive composition for cementing together atleast one metallized surface of a positive temperature coefficient (PTC)element to at least one metallic electrode, in the manufacture of PTCthermistor devices, and in which the adhesive component is essentially acurable silicone prepolymer, the improvement which comprises includingin the composition finely divided silicon carbide with finely-dividedsilicon, either separately or in admixture.

In another aspect, the present invention provides, in a method formanufacturing a positive temperature coefficient (PTC) thermistor devicewhich includes at least one step of cementing together at least onemetallized surface of a PTC element to at least one metallic electrode,by means of an electrically and thermally conductive adhesivecomposition in which the adhesive component is essentially a curablesilicone prepolymer the improvement which comprises including in thecomposition finely divided silicon carbide with finely-divided silicon,either separately or in admixture.

In still another aspect, the present invention provides a PTC thermistordevice which has been manufactured according to the method of theinvention. By utilizing the method of manufacture of the invention,there can be obtained PTC thermistor devices having the above-describeddesirable properties, and in particular, PTC thermistor devices whichare serviceable in a working temperature range not less than −55° C. to+300° C., and have a life of at least 30,000 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated fromthe following detailed description, taken in conjunction with thedrawings, in which:

FIG. 1A is a schematic side-sectional view of an electrical heatingdevice constructed by using an adhesive according to a particularembodiment of the present invention; and

FIGS. 1B and 1C are cross-sectional views of the electrical heatingdevice of FIG. 1A, taken along lines R—R and S—S therein, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In the electrically and thermally conductive adhesive compositionaccording to the invention, the adhesive component may be in general anycurable silicone prepolymer, known in the art for adhesive uses, andwhich may be cured in known manner by use of heat, moisture, catalyst,or other curing means, or any combination of the same (the preferredmanner of cure will generally depend on the nature of the siliconeprepolymer, in any particular case). The curable silicone prepolymer maybe for example a curable silicone rubber prepolymer, that is to say, itis a prepolymer which may be cured in a conventional manner, affording asilicone rubber-like product Examples of such prepolymers useful in thepresent invention, are those known under the codes KL-4 and KLT-30,which are manufactured by Synthetic Resin Plant,1 Lebedev Street, 420054Kazan, Russia; they are non-toxic, non-flammable and non-explosive, andotherwise have no adverse effects on humans; they are pastes, which arecolorless and white, respectively. Both of these prepolymer adhesivecomponents are used for sealing instruments to protect them fromatmospheric humidity and/or to protect them where they are subject tovibration, as well as for cementing glass (including organic glass),ceramics and other silicate-based materials, while KLT-30 is also usedfor sealing threaded connections, particularly in domestic plumbingsystems.

The adhesive composition of the invention is preferably characterized byat least one of the following features, namely:

(a) the metallized surface is selected from aluminized and silveredsurfaces;

(b) the at least one metallic electrode is at least one aluminumelectrode;

(c) the composition further includes a finely-divided metallic powder;

(d) the silicon carbide and (elemental) silicon are constituted by amixture which is a by-product of a step in the manufacture of siliconsemiconductors, which comprises polishing with silicon carbide, siliconplates or silicon wafers.

Moreover, the inventive composition is preferably additionallycharacterized by at least one of the following features, namely:

(i) the finely-divided metallic powder has a particle size no greaterthan about 40 μm;

(ii) said finely divided silicon carbide and finely divided silicon haveparticle sizes no greater than about 14 μm;

(iii) said finely divided silicon carbide and finely-divided silicon arepresent in a respective weight ratio of about 0.9 to 1.1: about 1.0;

(iv) the respective weight ratios of said finely-divided metallicpowder, said finely divided silicon carbide taken together with finelydivided silicon, and said curable silicone prepolymer, are 0.1 (±5%):1.1(±5%):1 (±5%), and preferably 0.1 (±1%):1.1 (±1%):1 (±1%).

It is also a particularly preferred feature of the adhesive compositionof the invention, that the curable silicone prepolymer has a viscosityat ambient temperature within the range of 15,000 to 25,000 μPa/sec.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1A, there is shown, as an illustration of theutility of the adhesive composition of the invention, an electricalheating device referred to generally as 10, which includes an array ofone or more positive temperature coefficient (PTC) thermistor heatingelements 11. Elements 11 are fabricated with preferably parallel,generally flat, surfaces on opposing faces 22, which are coated with aconductive metal such as aluminum, to serve as thermal and electricalcontact surfaces. On opposing sides of elements 11 there are locatedheat radiator units 20, each of which includes a plate 12 and separateseries of curved heat exchange fins 13 extending generally transverselyfrom each plate 12, respectively. Radiator units 20 are made of materialthat is a good thermal and electrical conductor, such as aluminum. Theplates 12 of the radiator units 20 are fabricated with flatinward-facing surfaces 24 to serve as thermal and electrical contactsurfaces. Pins 19 are provided in order to position plates 12 byengaging holes (not shown). Plates 12 may be fastened to pins 19 and apositioning frame (see below) by any suitable means, such as alloywelding or threaded screws or nuts. Plates 12 are moreover positioned sothat the inward-facing contact surfaces are generally parallel to and intouching contact with the outward-facing contact surfaces 22 of theheating elements 11, via intervening respective layers of cured adhesivecomposition according to the present invention. These cured adhesivelayers are intermediate each pair of surfaces 12, 22 and are notspecifically indicated in the Figures. Attached to the plates 12 areterminals 14 which allow the heating device 10 to be connected to anelectrical circuit. Direct application of electrical current to thethermistor heating elements 11 via the terminals 14 and the plates 12serves, inter alia, to minimize the number of components in the presentinvention, thereby simplifying its design. (it will be appreciated thateach radiator element, through which electricity is conducted to and viaPTC elements 11, may be regarded as an electrode; in other forms ofnomenclature terminals 14 are termed “electrodes”.)

The PTC thermistor heating elements 11 convert electrical energy,applied thereto, to thermal energy. The thermal energy is, in turn,conducted from the heating elements 11 to the heat exchange fins 13 viathe thermal interfaces and the plates 12. Air or other gas flow overfins 13 removes the heat from the device. As will be appreciated bypersons skilled in the art, the contact between the heating elements 11and the radiator units 20 via the highly heat- and electricallyconductive cured adhesive of the invention has the advantage of allowingthe heat to be transferred with great efficiency. This furthersimplifies the design of the device.

Enclosure of the array of heating elements 11 and the space containingit is shown in FIGS. 1B and 1C. FIG. 1B is a side-sectional view of theelectrical heating device of FIG. 1A, taken along line R—R therein,which cuts the device through one of the thermistor heating elements 11.FIG. 1C is a cross-sectional view of the electrical heating device ofFIG. 1A, taken along line S—S therein, which cuts the device through apair of spacers 17. It may be noted that the array of heating elements11 is completely enclosed by radiator unit plates 12 and an electricallyinsulating frame 15, the latter including longitudinal flanges 16A andend-pieces 16B, so that heating elements 11 are prevented from beingexposed to any cooling air or gas flow, so as to protect them from theknown “pinch effect.” For further details of the illustrated apparatus,reference may be made to our copending application no. (97102), theentire disclosure of which is deemed incorporated by reference herein.

The invention will be illustrated by the following non-limiting Example.

EXAMPLE

Materials

(a) KL-4 (Russian technical specification TU 38.103691-89) is a siliconeprepolymer which self-cures on exposure to atmospheric moisture and/orheat. It is normally stored in aluminum tubes, in which there is atendency for it to separate into layers, which does not affect itsutility; if, therefore, on squeezing 10 to 15 g from the tube, it isfound that separation into layers has occurred, the contents arethoroughly mixed before proceeding further. KL-4, when cured, isserviceable in the temperature range −60° C. to +300° C., has anelectrical strength of 15-25 kV/mm and a volume resistivity of4.5-5.5×10¹⁴ ohm-cm. A silicone prepolymer known by the code KLT-30 isalso suitable for the present purpose.

(b) 64SM14T (Russian technical specification TU 2-036-1005-87)extra-fine polishing powder of particle size 10-14 μm, an approximately1:1 by weight silicon carbide/silicon green admixture, in the form of a50% suspension in distilled water, is obtained as a by-product from astep of polishing silicon wafers with silicon carbide, in themanufacture of semiconductors. The suspension is allowed to settle in asettling tank and the water is drained off. The silicon carbide/siliconadmixture is heated at 120-130° C. for 2-3 hours. After cooling, thepolishing powder is screened through a brass sieve, mesh no. 004.

(c) Aluminum powder may be for example that known in Russia by the tradedesignation PAP-1, Russian government standard GOST 5494-71, particlesize 30-40 μm.

Preparation of the Adhesive Composition

Aluminum powder (10 pbw), KL-4 (100 pbw) and 64SM14T (110 pbw) arethoroughly mixed, preferably in a dry, inert atmosphere in a metallic,ceramic or inert polymeric vessel; depending on the efficiency of theblending operation, this need take no longer than 5-10 minutes, but itshould be verified that the product is homogeneous, and contains nounmixed pockets of aluminum. It is preferable to use the compositionimmediately, or in any event within eight hours.

Use of the Adhesive Composition

(a) The metallic electrodes, which are to be cemented to commerciallyavailable aluminized (or in the alternative, silvered) PTC elements, areconstituted by the inner planar surfaces of aluminum radiator elements,which are finned at the outer surface for more effective distribution ofgenerated heat, as described above with reference to the Figures. Theseinner surfaces are polished e.g. with a fine emery cloth, the residualduct from the polishing operation is removed, the surfaces are degreasedwith alcohol, and dried in air for 5-10 minutes. The homogeneousadhesive composition is applied evenly over the prepared surfaces, whichare then pressed together with an intervening metallized e.g. aluminizedPTC thermistor element (e.g. code B59102-R290-A10 of Siemens MatsushitaComponents) in a frame or clamp. Any excess adhesive is removed from theedges of the electrode/metallized PTC element/electrode assembly.

(b) The assembly is submitted to a pre-curing step of at least 16 hoursat ambient temperature in the range 15-35° C., while the pressure ismaintained, or increased, if necessary. Where a frame is used, this maybecome integral with the assembly.

(c) The assembly is then submitted to a curing step in a controlled-heatoven, wherein the temperature is raised by 40° C. until it reaches170±2° C., the temperature then being maintained at 170±2° C. for afurther four hours.

(Safety note: the usual precautions should be observed where handlingflammable liquids or vapors, aluminum powder, and acetic acid vaporswhich are evolved from the KL-4 component during the pre-curing and/orcuring reactions.)

The product of this Example complies with the above-stated requirements1-7. Tests on 150 such products over a 10,000 hour period (withoutfailure) demonstrate, as is known to practitioners in the art, that theyhave a 99% probability of a life expectancy exceeding 30,000 hours.

While particular embodiments of the invention have been particularlydescribed hereinabove, it will be appreciated that the present inventionis not limited thereto, since as will be readily apparent to skilledpersons, many modifications or variations can be made. Suchmodifications or variations which have not been detailed herein aredeemed to be obvious equivalents of the present invention.

1. In an electrically and thermally conductive adhesive composition for cementing together at least one metallized surface of a positive temperature coefficient (PTC) element to at least one metallic electrode, in the manufacture of PTC thermistor devices, and in which the adhesive component is essentially a curable silicone prepolymer, the improvement which comprises including in the composition finely divided silicon carbide and finely-divided silicon.
 2. An electrically and thermally conductive adhesive composition according to claim 1, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer, and said composition is additionally characterized by at least one of the following features, namely: (a) said metallized surface is selected from the group of aluminized and silvered surfaces; (b) said at least one metallic electrode is at least one aluminum electrode; (c) said composition further includes a finely-divided metallic powder; (d) said finely divided silicon carbide and finely-divided silicon are constituted by a mixture which is a by-product of a step in the manufacture of silicon semiconductors, which comprises polishing with silicon carbide, silicon plates or silicon wafers.
 3. An electrically and thermally conductive adhesive composition according to claim 1, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer, and said composition is additionally characterized by at least one of the following features, namely: (a) said metallized surface is selected from the group of aluminized and silvered surfaces; (b) said at least one metallic electrode is at least one aluminum electrode; (c) said composition further includes a finely-divided metallic powder selected from the group consisting of aluminum and silver powder; (d) said finely divided silicon carbide and finely-divided silicon are constituted by a mixture which is a by-product of a step in the manufacture of silicon semiconductors, which comprises polishing with silicon carbide, silicon plates or silicon wafers.
 4. An electrically and thermally conduct adhesive composition according to either claim 2 or claim 3, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer, and said composition is additionally characterized by at least one of the following features, namely: (i) said finely-divided metallic powder has a particle size no greater than about 40 μm; (ii) said finely divided silicon carbide and finely divided silicon have particle sizes no greater than about 14 μm; (iii) said finely divided silicon carbide and finely-divided silicon are present in a respective weight ratio of about 0.9 to 1.1: about 1.0; (iv) the respective weight ratios of said finely-divided metallic powder, said finely divided silicon carbide taken together with finely divided silicon, and said curable silicone prepolymer, are 0.1 (±5%):1.1 (±5%):1 (±5%).
 5. An electrically and thermally conductive composition according to claim 1, wherein said curable silicone prepolymer has a viscosity at ambient temperature within the range of 15,000 to 25,000 μPa/sec.
 6. An electrically and thermally conductive composition according to claim 4, wherein said curable silicone prepolymer has a viscosity at ambient temperature within the range of 15,000 to 25,000 μPa/sec.
 7. In a method for manufacturing a positive temperature coefficient (PTC) thermistor device which includes at least one step of cementing together at least one metallized surface of a PTC element to at least one metallic electrode, by means of an electrically and thermally conductive adhesive composition in which the adhesive component is essentially a curable silicone prepolymer, the improvement which comprises including in the composition finely divided silicon carbide and finely-divided silicon.
 8. A method according to claim 7, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer, and said composition is additionally characterized by at east one of the following features, namely: (a) said metallized surface is selected from the group of aluminized and silvered surfaces; (b) said at least one metallic electrode is at least one aluminum electrode; (c) said composition further includes a finely-divided metallic powder; (d) said finely divided silicon carbide and finely-divided silicon are constituted by a mixture which is a byproduct of a step in the manufacture of silicon semiconductors, which comprises polish with silicon carbide, silicon plates or silicon wafer.
 9. A method according to claim 7, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer, and said composition is additionally characterized by at least one of the following features, namely: (a) said metallized surface is selected from the group of aluminized and silvered surfaces; (b) said at least one metallic electrode is at least one aluminum electrode; (c) said composition further includes a finely-divided metallic powder selected from the group consisting of aluminum and silver powder; (d) said finely divided silicon carbide and finely-divided silicon are constituted by a mixture which is a by-product of a step in the manufacture of silicon semiconductors, which comprises polishing with silicon carbide, silicon plates or silicon wafers.
 10. A method according to either claim 8 or claim 9, wherein said curable silicone prepolymer is a curable silicone rube prepolymer, and said composition is additionally characterized by at least one of the following features, namely: (i) said finely-divided metallic powder has a particle size no greater than about 40 μm; (ii) said finely divided silicon carbide and finely divided silicon have particle sizes no greater than about 14 μm; (iii) said finely divided silicon carbide and finely-divided silicon are present in a respective weight ratio of about 0.9 to 1.1: about 1.0; (iv) the respective weight ratios of said finely-divided metallic powder, said finely divided silicon carbide taken together with finely divided silicon, and said curable silicone prepolymer, are 0.1 (±5%):1.1 (±5%):1 (±5%).
 11. A method according to claim 7, wherein said curable silicone prepolymer has a viscosity at ambient temperature within the range of 15,000 to 25,000 μPa/sec.
 12. A method according to either claim 8 or claim 9, wherein said curable silicone prepolymer is a curable silicone rubber prepolymer having a viscosity at ambient temperature within the range of 15,000 to 25,000 μPa/sec, and said composition is additionally characterized by at least one of the following features, namely: (i) said finely-divided metallic powder has a particle size no greater than about 40 μm; (ii) said finely divided silicon carbide and finely divided silicon have particle sizes no greater than about 14 μm; (iii) said finely divided silicon carbide and finely-divided silicon are present in a respective weight ratio of about 0.9 to 1.1:about 1.0; (iv) the respective weight ratios of said finely-divided metallic powder, said finely divided silicon carbide taken together with finely divided silicon, and said curable silicone prepolymer, are 0.1 (±5%):1.1 (±5%):1 (±5%).
 13. A PTC thermistor device which has been manufactured according to the method of any one of claims 7, 8, 9 or
 11. 14. A PTC thermistor device which has been manufactured according to the method of claim
 10. 15. A PTC thermistor device which has been manufactured according to the method of claim
 12. 16. A PTC thermistor device which has been manufactured according to the method of claim 7, is serviceable in a working temperature range not less than −55° C. to +300° C. and has a life of at least 30,000 hours.
 17. In an electrically and thermally conductive adhesive composition in which the adhesive component is essentially a curable silicone prepolymer, the improvement which comprises including in the composition finely divided silicon carbide with finely-divided silicon, either separately or in admixture, and a finely divided metallic powder. 